Recent evidence from the study of behavioral genetics has underscored the importance of the different or nonshared environments of siblings within the family. These data emphasize that it is these within family differences which are the most likely sources of non-genetic influence on both normal and pathological development in children and adolescents. This proposal focuses on the nonshared family environment which may contribute to the pathogenesis of psychopathology as well as the development of competence in adolescents using a novel step- family design. Step-families, depending on their form contains siblings who vary in genetic relationships from 50% for full sibs, to 25% for half sibs to 0% for blended sibs. This variation permits an estimate of which effects of the nonshared enviornment are genetically mediated and which are not. The design calls for a random and representative sample of 600 step-families equally divided into full, half, and blended sib types and a comparably sampled control group of 200 non-divorced families. Measures of the nonshared environment will include assessments of marital, parent-child, sibling, friendship and whole-family relationships. Of particular importance will be measurement by direct observation of family process and child behavior permitting an unusually precise estimate of both genetic and family interaction processes. Dependent measures will use current assessment procedures for conduct disorder and depression in adolescents as well as assessment of areas of competent development.